If you are over the age of 50, your primary care doctor has probably spoken to you about colon cancer screening. The purpose of colon cancer screening is to detect and remove pre-cancerous growths (polyps) in the large intestine. By doing this your likelihood of developing colon cancer is greatly reduced. Although colonoscopy [pronounced: ko-lun-AHS-ko-pee] has been the standard test for colon cancer screening, a new technology called CT colonography, also known as "virtual colonoscopy," is now being used in some people instead. A common question is how does this new test compare to traditional colonoscopy?

With CT colonography, a standard bowel preparation is given prior to the procedure to cleanse the colon. At the time of the test, a tube is inserted into the rectum and the colon is inflated with air. A CT scan x-ray is then taken of the colon. No sedation is given during this test. If any polyps or lesions are seen on the CT scan the patient then has to undergo a second procedure (traditional colonoscopy) for removal of the polyps. This usually is not done on the same day, so repeat bowel preparation and time off from work is necessary.

Traditional colonoscopy involves the same bowel preparation given the day prior to the procedure. On the day of the test the patient is given sedating medication so they are completely asleep during the procedure. A colonoscope, which is a tube with a light source and a camera, is inserted into the rectum and passed to the end of the colon. If any polyps or lesions are seen, they are removed or biopsied during the examination. All specimens are then sent to a pathologist for review under a microscope.

CT colonography detects medium and large polyps comparably to standard colonoscopy (if done using optimal technology). The major problem, however, is determining which patients with polyps on CT should be referred for colonoscopy with polyp removal. If the cut-off size is too large then some pre-cancerous polyps will be left in the colon and not removed. If the cut-off size is too small then too many patients would have to undergo both procedures.

In summary, with standard colonoscopy the patient undergoes a full bowel preparation, sedation is given for the exam, and it is a one-time exam with all polyps removed. It is the well established standard of care. In contrast, with CT colonography the patient undergoes the same full bowel preparation, a rectal tube is used to inflate the colon, and it remains unclear which patients with polyps should be referred for polyp removal. It is an emerging technology without a proven track record and not always covered by insurance companies or Medicare. It also involves radiation exposure during the CT scan.

CT colonography is an evolving technology that has potential for future colon cancer screening; however, the best option today is still traditional colonoscopy. Nevertheless, any form of colorectal cancer screening is better than no screening at all.

This article was published as part of the Daily Local News Medical Column series which appears every Monday. It has been reprinted by permission of the Daily Local News.